The invention relates to an auxiliary orthodontic appliance for human beings. This may be an auxiliary orthodontic or orthopaedic appliance for the maxilla or mandible. As used in technical terminology, such an auxiliary appliance is an article in the form of an expansion screw, a bracket, a buccal tube or a ring-shaped tooth band which remains in the mouth for a limited amount of time. Expansion screws are used for expanding the human jaw and also for correcting the position of one or several teeth. Brackets and buccal tubes also serve to correct the position of the teeth and are attached to the teeth, in the given circumstances, by means of a ring-shaped band which surrounds the pertinent tooth.
An expansion screw usually consists of two screw body parts and a threaded spindle which has threads running in opposite directions on either side of a spindle head at the center of the threaded spindle and engages corresponding threaded bores of the two screw body parts so that the two screw body parts are adjustable relative to one another by turning the threaded spindle. In the given circumstances, guide means, in particular, in the form of guide pins, can also be provided to prevent the two screw body parts from twisting in opposite directions when the threaded spindle is turned. The various parts of expansion screws known so far consist of stainless steel or nickel-silver alloys. Stainless steel was also used for the manufacture of brackets and buccal tubes, but ceramic and plastic brackets are also known, however, these are not fully satisfactory with respect to their strength.
Although it has been known for a long time that stainless steel and nickel-silver are not sufficiently resistant to corrosion for these purposes and, in particular, can cause allergic reactions on account of their nickel content, so far not a single manufacturer of such auxiliary appliances anywhere in the world has marketed expansion screws, brackets or buccal tubes which do not have the disadvantages explained above.
As the auxiliary appliances in question are worn in the mouth, the prevention of electrochemical corrosion of the auxiliary appliance constitutes quite a considerable problem. For example, in view of amalgam tooth fillings, the metallic auxiliary appliances known so far have also not proven fully satisfactory in this respect. In the case of expansion screws made of nickel-silver with guide pins and threaded spindles made of stainless steel, for example, the electrochemical corrosion can occur within a few days if the parts made of nickel-silver are not protected by a chrome or nickel coating.